A gyrometer of the type defined above has already been introduced and is described in the French Pat. No. 2 554 225. In this patent, the cavity has the form of a right-angled parallelepiped and is disposed so that two of its faces parallel to each other are perpendicular to the axis of the rotation to be measured. The first resonance mode, namely the mode excited by the excitation means, is a mode in which the particulate acoustic velocities are all parallel and orthogonal to the axis of rotation. The second resonance mode, namely the one induced by Coriolis forces, is of the same type, i.e. whose particulate acoustic velocities are all parallel, but the direction of the velocities of the induced mode is orthogonal to the direction of the velocities of the excitation mode. In order that such a gyrometer functions with good performance, it is important that a certain number of conditions be satisfied. These conditions are more especially the following:
the distances between two faces parallel to each other and orthogonal to a resonance mode must be strictly equal to a integer number of half-wave lengths at the excitation frequency, this number generally being the same and this imposes that two dimensions of the cavity are strictly equal, even if automatic control of the excitation frequency is provided,
the faces of the parallelepiped must be strictly parallel, and
the transducers allowing for measurement of the induced mode must strictly be positioned in a node point of the excitation mode.
Generally speaking, the above conditions are not truly satisfied. In particular, if the distance between the two faces orthogonal to the particulate acoustic velocities of the excitation mode is not rigorously equal to the distance between the two faces orthogonal to the particulate acoustic velocities of the induced mode, the natural frequency of the excitation mode shall slightly differ from the natural frequency of the induced mode. If the excitation frequency is controlled by an automatic frequency control in order to equal the natural frequency of the excitation mode, the induced mode shall be excited at a frequency slightly different from its natural frequency, which shall originate measuring inaccuracies. Thus generally speaking, the performances of such a gyrometer with a parallelepiped cavity are extremely sensitive to the effects of mechanical embodiment imperfections.